1. Field of the Invention
This invention relates in general to method of optical spectroscopy and a device for use in optical spectroscopy, and in particular to method of optical spectroscopy capable of robustly recovering an optical spectrum or spectra associated with a scene using Fabry-Perot transmission data and a device for use in such optical spectroscopy.
2. Description of the Related Art
Several standard methods to measure spectra exist, using dispersive elements, interferometers, optical filters, and/or micro-optical components and associated transforms. All of these spectral measurement methods have been applied to imaging systems, but none has achieved a compact device with acceptable broadband performance.
Drawbacks of using dispersive elements and Michelson interferometers are the space required to spread the spectrum across several elements of a detector and the size and weight of dispersive elements, beamsplitters and their mechanical mounts.
The primary disadvantage of using macroscopic size optical filters is related to size and weight. Mosaic array filters have been used to reduce the size and weight of optical filter instruments. These have been used in imaging devices, but with limited spatial and spectral resolution. There is an unfavorable tradeoff space for using mosaic array filters for imaging spectroscopy. The efficiency/resolution tradeoff available with optical filters also offers design challenges. Similarly, lens-mounted optical filter arrays lead to the same disadvantageous tradeoff between spatial and spectral resolution.
Micro-optical components have been successfully used to eliminate the size and weight disadvantages of other techniques and have achieved high spectral resolution. Two examples of the micro-optical devices are described as follows and have been used in line-scanning mode for imaging spectroscopy, requiring the image to be translated across one dimension of the micro-optic. These devices have achieved spectral measurement by exploiting the interference between reflections from surfaces of the micro-optic.
First, in the case where the reflectance of each surface of the micro-optic has been low, Fourier Transform algorithms have been used. Such a micro-optical device uses a wedged micro-optic to create an interference pattern on a focal plane. The low reflectivity of the micro-optic creates a sinusoidal modulation of the light falling on the detector surface. Low reflectance leads to low fringe contrast in the measured interference pattern, leading to poor signal-to-noise ratio performance under typical field conditions. An additional limitation of this device is that the Fourier Transform processing has a defined resolution that is tied directly to the maximum thickness of the wedged micro-optic. Therefore, for any device thickness, the number of resolved spectral elements within the bandwidth of the detector is fixed and cannot be adjusted. Since the Fourier Transform calculates all frequencies between the Nyquist frequency and zero, the usable number of resolved spectral elements may be significantly smaller than the number of distinct measurements on the detector.
Second, in the case where the micro-optic device used high reflectance, good results have been obtained only over a limited bandwidth. Multiplexing of the spectrum is not taken into account. The spectral range of these devices is therefore limited to a single free-spectral range (“FSR”) of the etalons. The spectral resolution of these devices is limited by the etalon finesse and the manufacturing tolerances for etalon arrays. The length difference between optical cavities in this device was on the order of a few nanometers. This type of device is difficult to manufacture, as step height must be accurately controlled, as each step is meant to provide unique information that is not duplicated by any other step. Further, the allowed usable step heights are limited to a few hundred nanometers, as in order to only be resonant with one optical frequency within the allowed bandwidth, the devices must have a FSR greater than the allowed bandwidth.
Another class of related devices is the scanning Fabry-Perot interferometer. Scanning Fabry-Perot interferometers require optical inputs with bandwidth less than one FSR of the device. This is due to the ambiguity between signals arising from colors separated by one FSR for any Fabry-Perot interferometer. Fabry-Perot theory is only valid within one FSR, and is not capable of demultiplexing mixed signals.